MURPHY and SHOMURA; PRE-EXPLOITATION ABUNDANCE OF TUNAS 



probably caused by the easterly flowing Equa- 

 torial Undercurrent (Figure 17) . It should also 

 be noted that each side of the equator has a pole- 

 ward component to the longline drift. These 

 observations confirm empirically the conclusion 

 by Cromwell (1953), based on theoretical con- 

 siderations, that divergence at the equator leads 

 to upwelling and that this enriched water moves 

 poleward. 



In the third panel (C), zooplankton is shown 

 to be most abundant at the center of upwelling, 

 falling off more precipitously to the north than 

 to the south. The zooplankton reaches a max- 

 imum at or near the equator, where the upwel- 

 ling is centered (panel A) , The model proposed 

 by Cromwell (1953) requires a net transport of 

 water away from the equator, and because the 

 zooplankton sampled is at least one trophic level 

 removed from chemical nutrients, we might ex- 

 pect its peak abundance to be significantly dis- 

 placed from the site of upwelling. The magni- 

 tude of the displacement would be a function of 

 the rapidity with which the zooplankton reaches 

 a peak, the speed of the north-south transport, 

 and possibly the rate of grazing. 



Since no displacement was observed in a 

 series of stations spaced at 96.5-km intervals, 

 the possible conditions include: (1) the zoo- 

 plankton develops very rapidly, (2) the north- 

 south transport is slow, (3) grazing is highest 

 just north and south of the equator, or (4) a 

 combination of the three factors. Nothing is 

 known of the development rate of equatorial zoo- 

 plankton (chiefly copepods), but some marine 

 cladocerans have been reported (Wickstead, 

 i963) to reach a peak in 36 to 40 days at tem- 

 peratures higher than 28 °C. Assuming this rate 

 of development for the equatorial zooplankton, 

 a poleward displacement of water as rapid as 

 6.4 km a day would be readily detected in plank- 

 ton hauls spaced 96.5 km apart. The lack of a 

 displacement suggests that grazing may be an 

 important factor controlling the spatial distri- 

 bution of the standing stock of zooplankton in 

 equatorial waters. 



In addition to the lack of horizontal displace- 

 ment of the peak of zooplankton from the equator 

 (Figure 18), the plankton volumes south of the 

 equator were slightly larger than those to the 



X 



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o 



200 



"1 — I — I — I — I — I — ] — ] — I — I — r— 

 (A) TEMPERATURE 'C 





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o 

 o 



I 



o 

 o 



(r 



UJ 



a. 



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o 



80 



60 



40 



20 





 20 



15 



10 



5 



"1 — I — I — r 



T \ — I — I — I — I — ] — 1 — \ — 

 (C) ZOOPLANKTON 



T — I — I — r 



1 — I — r 



1 — I — I — I — I — I — I — I — 

 (D) YELLOWFIN TUNA 



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J I L 



S 4° 3" 2° r 0° 1° 2° 3° 4° 5° 6° 7" 8° 9° 10" 1 1° 12° 13° 14° N 

 LATITUDE 



Figure 18. — Diagrams of several fields resulting from a 

 section along long 150°W in September 1951. A. Tem- 

 perature section from bathythermograms. B. Direction 

 of longline drift at each fishing station. C. Volumes of 

 zooplankton at each fishing station. D. Catch rate of 

 yellowfin tuna at each fishing station. (Adapted from 

 Sette, 1955.) 



north. This situation, however, is not the usual 

 one because King and Hida (1957) reported that 

 the average standing crop of plankton is greater 

 between lat 1.5°N and the southern boundary of 

 the Countercurrent (about lat 5°N) than be- 

 tween lat 1.5° and 5°S. This general situation 

 is thought to be the result of the dominance in 

 this region of the central Pacific southeast trades 

 which, according to Cromwell (1953), transport 

 most of the enriched water northward. In the 

 present instance, the longline drifts (Figure 18, 

 panel B) give evidence that the momentary ma- 

 jor transport was to the south of the equator. 

 Thus, the observed asymmetry of the plankton 

 is consistent with the momentary circulation. 



The yellowfin tuna catches (Figure 18, panel 

 D) show even less symmetry about the equator 



889 



